1. Field of the Invention
The present invention relates to a linear actuator.
2. The Prior Art
Actuators of this type are known i.e. from EP 531 247 A1, EP 586 326 A1, EP 647 799 and EP 662 573 A1 all of them to Linak NS. The forces of the spindle are transmitted to a compression/tension bearing embedded in a plastic housing. The forces between the bearing and the rear mounting are transmitted through the plastic housing which has to be dimensioned and shaped accordingly. Such a plastic housing represents a significant portion of the total cost of the actuator. WO 98/30816 to Linak NS discloses an example of a sophisticated actuator with very high performance in which the forces between the bearing and the rear mounting are transmitted via a metal chassis. The actuators are used for i.e. incorporation in furniture, e.g. hospital and nursing beds for which it is required that the actuators can withstand moisture and water. As for instance mentioned in EP 488 552 A1 Huntleigh Technology plc concerning a hospital bed. Since the actuators are fully or partly exposed it is an additional requirement that the actuators have a minimum of design with smooth and filth-repelling surfaces.
A non-design heavy-duty actuator without a housing for industrial purposes is based on a bearing metal housing with an integrated rear mounting from which the forces are transmitted.
An actuator of this type is known from DE GM 94 04 383.3 U1 with the housing being of die-cast zinc is equipped with a cylindrical portion in which the motor is located, said cylindrical portion being closed by means of a cover. Furthermore, the actuator is waterproof, if it is provided with appropriate sealing which makes it suitable for beds in the hospital and care sector. The die-cast zinc housing being relatively large as the motor has to be build in it, makes the solution relatively expensive.
EP 0 831 250 A2 Dana Corp discloses an actuator with a housing, having a shell which surrounds the motor and the transmission. The shell being open in the rear i.e. towards the rear mounting of the actuator is closed with a cover. In front, i.e. towards the spindle, the shell is equipped with a mounting for the outer tube which serves as guide for the spindle nut and as guide for the outer end of the tube shaped activation element being secured to the spindle nut. The lateral load which after all appear on the activation element and thus on the spindle are transmitted through the outer tube to the housing. However, it is also stated that the housing and the outer tube are made from a rigid material and as an example is stated i.e. aluminium, zinc alloys and stainless steel, i.e. the stated plastic materials: nylon containing glass fibre, polyvinylchloride and polyethylene all having a corresponding rigidness. All things considered a rather expensive solution.
WO 02/29284 A1 Linak A/S discloses an actuator in which a console is secured to the front part of the motor such that the electric motor and the console appear to be the bearing chassis of the motor while the housing is functioning as a dust and watertight cover, however, forces as such are not transmitted through the housing.
The object of the invention is to provide a compact actuator of the first mentioned type which is inexpensive to manufacture, but meets the requirements of these when it comes to output and quality.